1. Field of the Invention
This invention relates to liquid crystal cells and more particularly to a beam addressed liquid crystal cell having an absorbing layer adapted to absorb energy in the infrared spectrum.
2. Description of the Prior Art
Liquid crystal cell technology has advanced substantially during the past five years. Thermal copy papers and systems employing various heat sensitive liquid crystal cells have been investigated vigorously. Addressing liquid crystal cells thermally is attractive because the heat involved in the phase change is small compared to the latent heat of melting of typical organic compounds. The use of an infrared light beam for the local heating of liquid crystal cells is a frequently used approach. One serious problem with this approach, however, is the fact that known liquid crystalls are essentially transparent in the near infrared spectrum. As a result, the energy of the infrared light beam is not absorbed in the liquid crystal layer but in the much thicker glass envelope on either side of it. Typically, the glass envelope and liquid crystal cell together absorb only about ten to fifteen percent of the beam energy and most of this energy is lost by heat diffusion in the glass.
The use of additive fillers in liquid crystal compositions to absorb more of the infrared energy from the beam is described in U.S. Pat. Nos. 3,666,947, 3,499,702, 3,440,620, 3,409,404 and 3,411,513. The presence of suspended absorbers in liquid crystal cells has several disadvantages due to the effect on the properties of the liquid crystal. One problem pertains to the fact that these dyes usually absorb more in the visible spectrum than in the infrared spectrum. This fact results in the display becoming a monotone color thereby nullifying the gain in the absorption of the infrared energy. Another problem concerns the liquid crystal-dye dispersion itself. The presence of the dye in the liquid crystal material causes the resultant dispersion to have a grainy background if it is projected. Still another problem that may be encountered is a change in the electrical properties of the cell caused by the dyes which are usually salts.
Another approach is to use certain compounds such as the bis (dithobenzil) nickel complexes which have been found to absorb energy in the infrared spectrum but not in the visible spectrum. However, typically the absorption band is narrow and a solution of this compound in N-methylpyrrolidinone absorbs strongly at 940 nM but very little at 850 nM, the wavelength of gallium arsenide lasers. As a result, such a solution would not absorb sufficient energy from a gallium arsenide laser to adequately thermally address a liquid crystal cell.